The present invention relates to a gyrocompass of the type including a first gimbal, a gyro motor mounted in the first gimbal and including a gyro rotor mounted to rotate about a horizontally alignable spin axis, a pendulous body in which the first gimbal is mounted to be rotatable about a vertically alignable axis of rotation, a housing in which the body is rotatably suspended, a verticality device associated with the body for allowing the body to assume an orientation, in the housing in which the axis of rotation is vertical, and a follow-up device including a pickup mounted for sensing the angular position, about the axis of rotation, of the gimbal relative to the body and a drive connected for rotating the body relative to the housing about the axis of rotation in dependence on the position sensed by the pickup.
German Auslegeschrift [Published patent application] No. 1,498,042 discloses such a gyro which is designed in principle as a rate gyro with one degree of freedom. This prior art gyro includes a gimbal which is rotatable about an axis that is oriented as vertically as possible and a gyro rotor whose spin axis is orthogonal to the above-mentioned axis of rotation, or vertical axis, and thus lies in a horizontal plane. The gimbal is arranged in a so-called pendulous body in which it is rotatable on gas bearings, or cushions, and the pendulous body is suspended in a housing by means of a cardan, or universal, joint so that vertical alignment of the gimbal axis is possible.
The thus designed verticality adjustment device also includes, at the lower end of the pendulous body, an electrically actuated verticality magnet whose armature, after effecting the vertical alignment, is supported on a calotte-shaped part of the housing. The gimbal is restrained in a zero position with respect to the housing.
Moreover, a follow-up circuit is provided which includes a pickup for sensing the angular position of the gimbal, suitable amplifying means and a setting drive between the housing and the pendulous body so as to cause the pendulous body to precess in the sense of reducing the the gimbal pickup signal.
In such a gyrocompass the horizontal component of the earth's speed of rotation is known to induce a north driving moment, which is a function of the sine of the angle of displacement of the spin axis from the north direction, so as to cause a corresponding deflection of the gimbal. The follow-up circuit causes the pendulous body to follow until the above-mentioned deflection of the gimbal goes toward zero. The angular position of the pendulous body relative to the housing, as defined by an angle sensor in the form of a resolver, then essentially corresponds to the north direction. However, in addition to the north driving moment, there also exist the restraint moment and interference moments caused particularly by gyro drift, the gimbal bearing and current leads so that, depending on the quality of the gyro, there will occur greater or lesser north-seeking errors.
There additionally exists the requirement for a short north finding time, particularly since it is desired to locate north while the gyro carrier, for example a vehicle, is in a stationary state and, for tactical reasons, longer periods of immobility are generally undesirable for a vehicle.
German Offenlegungsschrift [Laid-open application] No. 2,336,956 additionally discloses a gyrocompass of the above-mentioned type in which there is initially effected a rapid follow-up into a first position in which the gyro spin axis is roughly oriented in the north direction. Thereafter, the follow-up circuit is cut off and a torque generator, or torquer, disposed between the gimbal and the pendulous body is actuated in dependence on the gimbal pickup signal. In the restraint circuit of this design, the current of the torquer then corresponds to the restraining moment so that, under consideration of the angular moment imposed on the gyro, the input angular velocity, and thus the existing deviation from north, can be determined.
Suitable filters are provided to limit the influx of the expected external interference moments.
There also is provided an angle sensor to determine the angular position of the pendulous body with respect to the housing and the signal from this angle sensor and the north deviation signal provide the north direction indication. By including the above-mentioned restraint circuit, and with the corresponding given transfer factor, the dominant natural frequency of the gyro is increased so that the north error as a result of rectifying moments or the sensitivity to interfering movements is reduced, respectively. But since, on the one hand, the restraint moment depends on the gyro drift, which changes in an unpredictable manner, particularly with temperature or time, the emitted north angle indication may contain not insignificant errors.